Acoustic telemetry is a method of communication in the well drilling and production industry. In a typical drilling environment, acoustic carrier waves from an acoustic telemetry device are modulated in order to carry information via the drillpipe to the surface. Upon arrival at the surface, the waves are detected, decoded and displayed in order that drillers, geologists and others helping steer or control the well are provided with drilling and formation data.
The theory of acoustic telemetry as applied to communication along drillstrings has a long history, and a comprehensive theoretical understanding was eventually achieved and backed up by accurate measurements. It is now generally recognized that the nearly regular periodic structure of drillpipe imposes a passband/stopband structure on the frequency response, similar to that of a comb filter. Dispersion, phase non-linearity and frequency-dependent attenuation make drillpipe a challenging medium for telemetry, which situation is made even more challenging by the significant surface and downhole noise generally experienced.
The design of acoustic systems for static production wells has been reasonably successful, as each system can be modified within economic constraints to suit these relatively long-lived applications. The application of acoustic telemetry in the plethora of individually differing real-time drilling situations, however, is much less successful. This is primarily due to the increased noise due to drilling, and the problem of unwanted acoustic wave reflections associated with downhole components, such as the bottom-hole assembly (or ‘BHA’), typically attached to the end of the drillstring, which reflections can interfere with the desired acoustic telemetry signal. The problem of communication through drillpipe is further complicated by the fact that drillpipe has heavier tool joints than production tubing, resulting in broader stopbands; this entails relatively less available acoustic passband spectrum, making the problems of noise and signal distortion more severe.
To make the situation even more challenging, BHA components are normally designed without any regard to acoustic telemetry applications, enhancing the risk of unwanted and possibly deleterious reflections caused primarily by the BHA components.
When exploring for oil or gas, or in coal mine drilling applications, an acoustic transmitter is preferentially placed near the BHA, typically near the drill bit where the transmitter can gather certain drilling and formation data, process this data, and then convert the data into a signal to be broadcast to an appropriate receiving and decoding station. In some systems, the transmitter is designed to produce elastic extensional stress waves that propagate through the drillstring to the surface, where the waves are detected by sensors, such as accelerometers, attached to the drill string or associated drilling rig equipment. These waves carry information of value to the drillers and others who are responsible for steering the well.
Exploration drilling in particular has become a highly evolved art, wherein the specification and placement of the BHA components is almost entirely dictated by the driller's need to drill as quickly and accurately as possible while gathering information local to the drill bit. A large variety of specialized BHA modules or tools are available to suit local conditions, and their inclusion in a BHA usually takes priority over the requirements of telemetry methods, acoustic or otherwise. The diversity of these BHA tools and the decision regarding whether or not to even include them in a drillstring pose major issues for consideration; these issues have a significant impact when dealing with acoustic energy questions. Cyclic acoustic waves suffer multiple reflections and amplitude changes even in a very simple BHA, and the net effect of these changes may destructively interfere with the required acoustic telemetry broadcast signal. The reflections are caused by impedance mismatches, which are the result of mechanical discontinuities present in all BHAs presently in use.
An initial response to this problem would be to place the acoustic telemetry device above the BHA and simply direct the acoustic energy up the drillstring, away from the BHA components. Unfortunately, this does not fully address the problem because typical acoustic transmitters emit waves of equal magnitude both up-hole and downhole, and the downward travelling waves in particular may be reflected resulting in destructive interference with the upward travelling waves. In the worst cases, this can cause virtually complete cancellation of the upward travelling communication signal.
It is known in other fields, for example in radio frequency transmitter design and electrical transmission lines, that wave reflections can be controlled by inserting simple specific impedance changes at certain distances from a transmitter, such that the combination of the original wave and the reflected wave combine constructively to produce a single wave travelling in one direction with increased amplitude. The appropriate technique in our context is to insert a ‘quarter wave’(λ/4, λ being a wavelength) impedance change (or odd integer multiples thereof, i.e. noddλ/4) adjacent to the transmitter so that one wave (the ‘down’ wave) is reflected in phase with the intended transmitted wave (the ‘up’ wave) and constructively aids the intended transmitted wave by increasing its amplitude.
Downhole applications typically employ transmitters that emit stress waves of nearly equal, but not necessarily equal, magnitude in both directions. Moreover, each wave has the same sign in stress but opposite sign in material velocity. In such cases, the appropriate reflection device would be a noddλ/4 tuning bar placed below the transmitter. However, such a simple solution is often impractical because the equipment below the acoustic transmitter is designed to drill and steer the well rather than to aid telemetry. Equipment such as drill collars, crossover pipes, drilling motors and bits can easily nullify the benefit of simply introducing a noddλ/4 section of pipe below the acoustic transmitter because the equipment will generally be of differing lengths and impedances that can add to the noddλ/4 section and eliminate the intended benefit.
Other styles of transmitters, which emit waves in both directions having different relationships between their stresses and material velocity, would require tuning bars of different lengths, not necessarily noddλ/4 sections, further complicating the problem.
As mentioned above, downhole noise is also of concern in acoustic telemetry. The problem of downhole noise is addressed to some extent in U.S. Pat. No. 6,535,458 to Meehan, wherein is taught a baffle filter comprising a periodic structure of typically 20 m length interposed above or below the acoustic sub; this is intended to cause stopbands over a certain range of frequencies, the position of the baffle being to protect the acoustic transmitter from the sources of the noise. This teaching, however, does not address or anticipate the more serious problem of energy propagating in a ‘down’ direction being reflected in a relatively unattenuated manner back to the transmitter where it may combine in a destructive manner with the energy propagating in an ‘up’ direction, thereby causing possibly significant destruction of the signal intended to reach the surface.
As can be seen, then, the required upward travelling acoustic telemetry waves are often interfered with by unwanted reflections from impedance mismatches below the transmitter. The known art of inserting a tuning bar of appropriate length is usually ineffective because the local conditions often necessitate the addition of further BHA components that cause further reflections that can often destructively interfere with the upward travelling wave.